Design of a Comfortable Pure Moment Knee Exoskeleton

Lower-limb exoskeletons have the potential to enhance rehabilitation [1], assist in walking with gait impairments [2], reduce the metabolic cost of normal [3] and load-bearing walking [4], improve stability [5] and probe interesting questions about human locomotion [6]. The ankle produces larger peak torques and performs more positive work than either the hip or the knee during locomotion [7], and this makes it an effective location for applying assistance through an exoskeleton. However, it is difficult to assist a single joint without affecting other joints. Active plantarflexion causes a forward shift in ground reaction force, which extends the knee. This extension torque is resisted by the gastrocnemius as it acts as both a plantarflexor and a knee flexor. Forces in the gastrocnemius may be reduced when applying external torque to the ankle during push off, while knee extension torque resulting from ground reaction forces remains unchanged. This may cause hyperextension of the knee or increased activity of other knee flexors, thereby reducing the benefits of using an ankle exoskeleton. A knee exoskeleton may therefore be useful not only alone, but also in conjunction with an ankle exoskeleton to resist hyperextension during push off.